Equine embryo cryopreservation is unsuccessful in larger, more easily collected, day-7 embryos. It is imperative that methods to successfully cryopreserve large equine embryos or develop reliable methods to determine embryo size before collection. Therefore the objectives for this study were to quantify the amount of tritiated glycerol that would permeate various sizes of equine embryos and to determine if circulating progesterone concentration was correlated with in utero embryo size. Mean embryo diameter (▒ SEM) across treatments (1.4M and 3.4M tritiated glycerol) was 696.5Ám ▒ 108.6Ám and 925.9 Ám ▒ 214.1Ám, respectively and were not different (P=0.44). The percent permeation for 1.4M and 3.4M glycerol were not different (P=0.68). Embryos <400 Ám in the 1.4M glycerol treatment group had higher (P=0.002) permeation than embryos >400 Ám, 8.32% ▒ 3.85% and 0.35% ▒ 0.11%, respectively. Length of time, 60 or 120 minutes, did not affect amount of glycerol uptaken (P=0.26. Serum progesterone concentrations on day 7 post-ovulation were higher (P=0.009) for mares who produced two viable embryos from double ovulation (24.17▒2.82ng/ml) compared with mares from which a single embryo (14.04▒0.99ng/ml) was collected and control mares (13.53▒1.80ng/ml). No differences (P=0.91) were detected in serum progesterone concentration on day 7 post-ovulation between mares from which a single embryo (14.04▒0.99ng/ml) was collected and control mares (13.53▒1.80ng/ml). Mares producing embryos >400Ám tended to have higher (P=0.08) circulating progesterone concentrations than mares producing embryos <400Ám. Serum progesterone concentrations day 7 post-ovulation in mares producing embryos >400Ám and <400Ám were not different (P=0.61 and P=0.68, respectively) than control mares. Single embryos <1000Ám in diameter were correlated with circulating progesterone concentration day 7 post-ovulation (r=0.46, P=0.006). There was no significant correlation between embryo diameter, corpus luteum diameter, and serum progesterone concentration day 7 post-ovulation.This is the first study to quantify the amount of glycerol permeating into equine blastocysts and suggests that the capsule may be a barrier to cryoprotectant permeability. Maternal progesterone concentrations day 7 post-ovulation could be utilized in predicting embryo stage and size prior to collection for cryopreservation and in diagnosis of twin pregnancies as a result of double ovulation.